Find yourself on floor 3 here (red arrow).

Go to this room, bring down the guard by the window with your knife and get three bullets.

The moment you open fire the sub mutant in this room, the Super SS in the purple room to the north and the guard in the little room to the south will be alarmed. Finish the sub mutant, find a full ammo clip. Take on the Super SS and the guard as they come. The Super SS will leave you a plasma gun.

Find two gold chalices in the bathroom. Tin chalices are found in other places.

Use your plasma gun to shoot the armour in this room. You get an axe.

Mow down all the sub mutants in this room with your axe. Open the scret door and get the cross.

Get more health. Go down to floor 2 (green arrow).

Continue.